1. Field of the Invention
The present invention concerns a sheath wave barrier unit for an outer shielding (jacket) of a coaxial cable that also has an inner conductor, of the type having first and second sheath wave barriers, the first sheath wave barrier damping or suppressing sheath waves that are induced in a first conductor segment of the outer shielding and the second sheath wave barrier damping or suppressing the sheath waves that are induced in a second conductor segment of the outer shielding that is in series with the first conductor, wherein each sheath wave barrier forms a resonant oscillator circuit at a predetermined high frequency with the high frequency being the same for both sheath wave barriers.
2. Description of the Prior Art
Sheath wave barriers are used in the feed lines and return lines of local coils of magnetic resonance systems. They serve to damp to suppress sheath waves (standing waves) that would otherwise be induced in the outer shieldings of these lines due to the strong radio-frequency fields used for the excitation of magnetic resonances without the sheath wave barriers. Normally a number of sheath wave barriers are present in each supply or return line, the sheath wave barrier unit being of the type described above. German PS 41 13 120 describes examples of this prior art.
An electrical signal filter having two filter circuits that are decoupled from one another by a shielding device is known from U.S. Pat. No. 5,432,488. The shielding unit has a radial shield, a tangential shield arranged on the radial shield, as well as annular shields. The tangential shield and the annular shields essentially completely encapsulate the filter circuits. The radial shield decouples the filter circuits from one another. The filter circuits can be connected with the inner conductor of a coaxial cable via an input connector and an output connector.
The basic magnetic field of the magnetic resonance system is normally 0.2 to 1.5 Tesla in conventional systems. The magnetic resonance frequency corresponding with this field for the detection of hydrogen (which is the most common operational use) is approximately 8.5 to 63.5 MHz. At these magnetic resonance frequencies, the individual sheath wave barriers can be separated from one another by a distance such that they barely mutually influence one another.
Magnetic resonance system also are known in which the basic magnetic field is greater than 1.5 Tesla, sometimes 2.5 Tesla and more. The magnetic resonance frequency has increases to over 100 MHz. At this frequency, a significantly stronger excitation of sheath waves occurs in the outer shielding of the feed and leakage lines. More sheath wave barriers therefore must be used, so the distance between the individual sheath wave barriers is reduced and as a result, an unwanted mutual influencing between barriers occurs. Due to the stronger excitation of sheath waves, the sheath waves must be more strongly damped, such that the voltage load, the current load and the thermal load of the sheath waves increase.